People reversed their biological age by almost 3 years on a diet that mimics fasting in a study. Here's what they ate.

• A diet that mimics fasting appeared to reverse people's biological age by 2.5 years in a study.  
• Participants who followed the regime had less abdominal fat and healthier immune cells. 
•  Fasting is not for everyone and the diet is harsh, an expert said. 

Following a diet that mimics fasting for five days a month appeared to reverse people's biological age by two and a half years on average in a study.

The fasting-mimicking diet, developed by the lab of senior study author Valter Longo at USC Leonard Davis School of Gerontology, is designed to mimic the effects of a water-only fast while providing necessary nutrients, a press release said. Water fasting involves drinking only water for 24 to 72 hours.

Fasting is not safe for everyone, but studies, mostly on cells and animals, suggest it may have health benefits, such as lowering the risk of certain cancers, type 2 diabetes, and cardiovascular disease.

Fasting or restricting calories is thought to induce what is known as autophagy, which is the body's process of clearing out old, damaged cells to make space for new ones. It begins when cells are stressed or deprived of nutrients and naturally decreases with age, meaning it plays an important role in slowing aging, according to The Cleveland Clinic.

In the study, around half of the 100 participants were told to follow the FMD for five days a month for three months, and to eat their regular diet for the other 25 days. The other group just ate their normal diet. Participants were men and women aged between 18 and 70.

Blood tests and MRI scans of the participants showed those in the FMD group were less likely to develop diabetes, and had lower levels of abdominal and liver fat, which are associated with a reduced risk of metabolic syndromes, such as liver disease, cardiovascular disease, and diabetes, the study said. It also increased their lymphoid-to-myeloid ratio — an indicator of a more youthful immune system. These factors can be used to measure biological age, the authors of the study published January 20 in Nature Communications, said.

While chronological age refers to how many years someone has been alive, biological age is a measure of how well cells and tissues are functioning. However, the term is still debated among experts as younger cells may not necessarily indicate better health, and there is no "normal" way our cells should look at any given age, BI previously reported.

Participants could eat foods including soup and energy bars while 'fasting'

The FMD is a low-calorie diet that's high in unsaturated fats and low in protein and carbohydrates. During the five-day-fast period, participants consumed vegetable-based soups, energy bars, energy drinks, chip snacks, tea, and a supplement providing high levels of minerals, vitamins, and essential fatty acids, provided by L-Nutra, a nutrition tech company founded by Longo. On day one of the FMD, participants ate around 1,100 calories, and around 720 on days two to five. They could eat at any time throughout the day.

Longo said in a press release: "This is the first study to show that a food-based intervention that does not require chronic dietary or other lifestyle changes can make people biologically younger, based on both changes in risk factors for aging and disease and on a validated method developed by the Levine group to assess biological age."

However, the study involved only 100 participants, the majority of whom were healthier than the average person in the US population, the authors said, meaning the results might not relate to a wider population.

"It would need to be rolled out to lots more people in different demographic and health groups to really understand how beneficial fasting is for everyone," professor Clare Bryant, who researches innate immunity at Cambridge University, UK, and was not involved in the study, told Business Insider.

Previous studies have linked the FMD to stem cell regeneration as well as reducing the side effects of chemotherapy in studies on mice.

David Clancy, a lecturer who studies the biology of aging at Lancaster University, UK, who was not involved in the study, told BI: "It's not unreasonable to think that, during ages 40 to 60 at least, this regime twice per year may add three to four years of healthy life, maybe more, in those with higher BMI, blood pressure, blood sugar etc."

But he noted that the diet is "pretty harsh," and people may struggle to get through the working day towards the end. "Scheduling days four and five for weekend days would be sensible," he said.

Cultivating a sensible exercise habit could achieve the same or even better results, Clancy said, and is maintainable into older age.